Microcephaly is a neurological disorder in which the circumference of the head is significantly smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life. The disorder may stem from a wide variety of conditions that cause abnormal growth of the brain, or from syndromes associated with chromosomal abnormalities. A mutation of the ASPM gene causes autosomal recessive primary microcephaly.

Infants with microcephaly are born with either a normal or reduced head size. Subsequently the head fails to grow while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed. Development of motor functions and speech may be delayed. Hyperactivity and mental retardation are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from clumsiness in some to spastic quadriplegia in others.

Generally there is no specific treatment for microcephaly. Treatment is symptomatic and supportive.

In general, life expectancy for individuals with microcephaly is reduced and the prognosis for normal brain function is poor. The prognosis varies depending on the presence of associated abnormalities.

Micrencephaly is a type of cephalic disorder. This is a disorder characterized by a small brain and may be caused by a disturbance in the proliferation of nerve cells. Micrencephaly may also be associated with maternal problems such as alcoholism, diabetes, or rubella (German measles). A genetic factor may play a role in causing some cases of micrencephaly. Affected newborns generally have striking neurological defects and seizures. Severely impaired intellectual development is common, but disturbances in motor functions may not appear until later in life.